1. Field of the Invention
The present invention relates to a modulation circuit for use in a digital radio communications apparatus, such as a digital portable telephone apparatus, a digital automobile telephone apparatus and a digital cordless telephone apparatus and, in particular, to a modulation circuit using a phase shifted DQPSK modulation system as a modulation system.
2. Description of the Related Art
Recently, proposals has been advanced to adopt a digital apparatus as one of radio telephone systems. This type of system can transmit not only a control signal between a mobile station and a a base station but also communication items, such as a tone of speech, in a digital form. This system is of advantage in that it can achieve improved privacy, improved compatibility with data, effective utilization of a radio frequency, etc.
A digital radio communications equipment in this type of system uses, as one modulation system for example, a .pi./4 shifted DQPSK (.pi./4 shifted, differentially encoded quadrature phase shift keying) modulation system. In a modulation circuit using this type of .pi./4 shifted DQPSK modulation system, a transmit data stream is converted by a serial/parallel converter into two data streams X.sub.k, Y.sub.k. These data streams x.sub.k and Y.sub.k are differentially phase coded, by a differentially coded mapping circuit, to a form as given below: EQU I.sub.k =I.sub.k-1 cos [.DELTA..phi.(X.sub.k,Y.sub.k)]-Q.sub.k-1 sin [.DELTA..phi.(X.sub.k,Y.sub.k)] EQU Q.sub.k =I.sub.k-1 sin [.DELTA..phi.(X.sub.k,Y.sub.k)]+Q.sub.k-1 cos [.DELTA..phi.(X.sub.k,Y.sub.k)]
where
I.sub.k-1, Q.sub.k-1 denote the amplitudes of coded data at a pulse time corresponding to one-previous pulse time, and .DELTA..phi. denotes an amount of phase change.
FIG. 7 shows a relation of input data streams x.sub.k, Y.sub.k to .DELTA..phi. and FIG. 8 shows a phase space diagram representing the phase mapping positions of coded data I.sub.k, Q.sub.k output from the differentially phase coded mapping circuit.
That is, the differentially coded mapping circuit determines the phase matching position of the coded data I.sub.k, Q.sub.k at each pulse time on the basis of the signal levels ("1", "0") of the input data streams X.sub.k, Y.sub.k and the phase mapping position of the coded data I.sub.k-1, Q.sub.k-1 at the pulse time corresponding to one-previous pulse time. At each pulse time, any of the positions as indicated by ".largecircle." and any of the positions " " in FIG. 8 are alternately selected on the phase mapping position. Further, it is possible to selectively take, as the amplitude value of the coded data I.sub.k, Q.sub.k, one of five values: 0, .+-.1, .+-.2.sup.1/2 (.sqroot.2).
The use of such .pi./4 shifted DQPSK modulation system can suppress the spreading of a signal band involved.
However, no practical modulation circuit-incorporated arrangement has yet been achieved.